Ceramic honeycomb filter and its production method

ABSTRACT

A ceramic honeycomb filter comprising a ceramic honeycomb structure having pluralities of flow paths partitioned by porous cordierite cell walls, and plugs formed in predetermined flow paths of the ceramic honeycomb structure; the plugs being formed by ceramic particles and an amorphous oxide matrix existing between the ceramic particles; in a cross section of the plugs, an area ratio A 1  of the amorphous oxide matrix in a longitudinal range of ⅓×t from one end, and an area ratio A 2  of the amorphous oxide matrix in a longitudinal range of ⅓×t from the other end meeting the relation of ½≦A 1 /A 2 ≦2, wherein t represents the length of the plug in a direction perpendicular to the longitudinal direction of the plug.

FIELD OF THE INVENTION

The present invention relates to a ceramic honeycomb filter for removingparticulate matter from an exhaust gas discharged from diesel engines,and its production method.

BACKGROUND OF THE INVENTION

Investigation has been conducted to remove particulate matter dischargedfrom diesel engines, by using particulate-matter-capturing ceramichoneycomb filters comprising ceramic honeycomb structures having porouscell walls, through which an exhaust gas containing particulate matterpasses, namely diesel particulate filters (DPFs). As shown in FIG. 1,the ceramic honeycomb filter 11 comprises pluralities of flow paths 15a, 15 b partitioned by porous cell walls 14, a peripheral wall 11 aformed around the flow paths 15 a, 15 b, and plugs 13 a, 13 balternately formed in end portions of the flow paths 15 a, 15 b. Asshown in FIG. 1(b), an exhaust gas containing particulate matter flowsinto the flow paths 15 b open on an exhaust gas inlet-side end surface12 a, passes through cell walls 14, and flows out of the flow paths 15 aopen on an exhaust gas outlet-side end surface 12 b, during whichparticulate matter in the exhaust gas is captured by fine pores (notshown) in the cell walls 14.

When the captured particulate matter is excessively accumulated in aceramic honeycomb filter, the pressure loss of the filter increases,likely resulting in power decrease of the engine. Accordingly, thecaptured particulate matter is periodically burned by an externalignition means such as an electric heater, a burner, etc., to regeneratethe ceramic honeycomb filter. A set of two ceramic honeycomb filters areusually mounted for an alternate regeneration system, in which one isused while the other one is regenerated.

With respect to the characteristics of a honeycomb filter having theabove structure, it is important to keep the pressure loss of a filterlow to avoid decrease in engine performance. It is also required thatthe honeycomb filter has enough resistance to withstand heat shock dueto rapid temperature change during regeneration, engine stop, etc. Thus,technologies for improving plugs of ceramic honeycomb filters have beendisclosed so far, as described below.

As a technology of plugging end surfaces of a ceramic honeycombstructure at predetermined positions, JP 63-28875 B discloses a methodfor plugging open end surfaces of a ceramic honeycomb structure,comprising plugging ends of flow paths in a sintered honeycomb structurewith a cordierite-forming material paste, and then sintering thecordierite-forming material to cordierite at a temperature of 1300° C.or higher. This method achieves complete plugging of flow paths of aceramic honeycomb structure at open end surfaces, providing a highlyreliable cordierite honeycomb filter having excellent heat shockresistance.

JP 2002-136817 A discloses a ceramic honeycomb filter obtained byplugging flow path ends of a sintered or unsintered ceramic honeycombstructure with a pulverized sintered or unsintered plugging material,which has the same composition as that of the ceramic honeycombstructure, and heating the plugging material at a high temperature of1400° C. to form plugs at flow path ends of the ceramic honeycombstructure. It further describes that because plugs at flow path ends ofthis ceramic honeycomb filter are made of the same material as that ofthe ceramic honeycomb structure, the ceramic honeycomb structure and theplugs do not suffer cracking due to their thermal expansion differenceand are free from troubles such as the peeling of plugs, when used underhigh-temperature conditions.

However, the ceramic honeycomb structure likely has a small thermalexpansion coefficient because a cordierite-forming material is orientedby extrusion, while a cordierite-forming material is not substantiallyoriented in plugs. Accordingly, in the technologies described in JP63-28875 B and JP 2002-136817 A, it is difficult to provide the ceramichoneycomb structure and the plugs with completely the same thermalexpansion coefficient. Further, because the plugs are fused to thesintered ceramic honeycomb structure at high temperatures of 1300° C. orhigher, large residual stress is generated after fusion. Thus, heatshock by an exhaust gas, and mechanical shock by engine vibration androad vibration, cracking occurs in plugs, their boundaries with thehoneycomb structure, etc., likely resulting in the peeling of plugswhile using the filter.

To solve such problems, JP 2005-125318 A discloses a ceramic honeycombfilter comprising porous cell walls defining flow paths for removingparticulate matter from an exhaust gas, which is obtained by formingplugs in predetermined flow paths of a ceramic honeycomb structure madeof a material comprising cordierite as a main crystal; at least part ofthe plugs being formed by at least ceramic particles and an amorphousoxide matrix of colloidal oxide. JP 2005-125318 A describes that thishoneycomb filter is obtained by bonding the plugs to the ceramichoneycomb structure at 1000° C. or lower. According to this invention,there is little difference in a thermal expansion coefficient betweenthe plugs comprising at least ceramic particles and the ceramichoneycomb structure, and a bonding temperature lowered by using anamorphous oxide matrix of colloidal oxide leaves less residual stress inthe ceramic honeycomb structure, resulting in a ceramic honeycomb filterhaving excellent heat shock resistance. In addition, the low bondingtemperature is effective to drastically reduce a production cost.

Though the ceramic honeycomb filter of JP 2005-125318 A used as aparticulate-matter-capturing filter has excellent heat shock resistance,it has been found when it is used as a ceramic honeycomb filter carryinga catalyst such as an oxidation catalyst for accelerating the oxidation(combustion) of captured particulate matter, which may be called“catalyst-carrying filter” below), the temperature of a filter substrateis elevated by combustion, reducing the bonding strength of the plugs tocell walls, so that the plugs may be detached, resulting in lowparticulate-matter-capturing performance.

JP 2015-505748 A discloses a method for forming plugs hardenable at lowtemperatures without sintering by charging a aqueous compositioncomprising a refractory filler comprising coarse cordierite particleshaving a narrow particle size distribution with d₅₀ of 10-40 μm, aninorganic binder, and a binder, into a ceramic honeycomb body. Itdescribes that the plugs are provided with less recesses when theaqueous composition charged into the ceramic honeycomb body is dried. JP2015-505748 A describes that larger particle sizes in the filler reduceshrinkage and the overall movement of a composition in flow paths orfine pores, thereby reducing the number of recesses.

However, it has been found that the method described in JP 2015-505748 Adoes not provide sufficient bonding strength, when the aqueouscomposition charged into the ceramic honeycomb body is dried, forexample, in a hot-air furnace.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide a ceramichoneycomb filter, in which plugs formed at 1000° C. or lower have goodbonding strength to cell walls, and a method for producing such aceramic honeycomb filter at a low plug-bonding temperature withdrastically reduced production cost.

SUMMARY OF THE INVENTION

In view of the above object, the inventors have made intensive researchon why plugs formed by at least ceramic particles and an amorphous oxidematrix of colloidal oxide have insufficient bonding strength to cellwalls of a honeycomb filter, resulting in the detachment of plugs duringuse, finding that such problem is caused by the following phenomena whenthe plugs are bonded by heating and drying.

When a plugging material comprising ceramic particles and colloidaloxide is charged into flow path ends of a ceramic honeycomb structure,and dried by heating at a temperature of 1000° C. or lower, for example,in a hot-air furnace, water is evaporated from a flow path end surfaceside on which the plugging material is charged, because such side isexposed to hot air. With water evaporated from the flow path end surfaceside, water moves from the plugging material on the side opposite to theflow path end surface side (flow path inner side) to the flow path endsurface, accompanied by the movement of colloidal oxide from the flowpath inner side to the flow path end surface side. As a result, theconcentration of colloidal oxide decreases on the flow path inner side,and increases on the flow path end surface side, resulting in largedifference in the concentration of colloidal oxide between the flow pathinner side and the flow path end surface side. Thus, the bonding ofplugs on the flow path inner side is insufficient, so that the plugshave substantially shorter lengths, and thus insufficient bondingstrength to cell walls.

As a result of further intensive research, the inventors have found thatby drying the charged plugging material by microwave heating orhigh-frequency dielectric heating, the entire plugs are uniformlyheated, resulting in substantially no difference in the concentration ofcolloidal oxide between the flow path end surface side and the oppositeside. Because plugs and cell walls are uniformly bonded at anylongitudinal position, a ceramic honeycomb filter having sufficientbonding strength between plugs and cell walls can be obtained. Thepresent invention has been completed based on such findings.

Thus, the ceramic honeycomb filter of the present invention comprises aceramic honeycomb structure having pluralities of flow paths partitionedby porous cell walls made of a material comprising cordierite as a maincrystal, and plugs formed in predetermined flow paths of the ceramichoneycomb structure; the plugs comprising ceramic particles and anamorphous oxide matrix existing between the ceramic particles;

the amorphous oxide matrix being 5-20 parts by mass per 100 parts bymass of the ceramic particles; and

in a cross section of the plug including the center axis of the flowpath, a ratio A1/A2 meeting the relation of ½≦A1/A2≦2, wherein A1represents an area ratio of the amorphous oxide matrix in a longitudinalrange of ⅓×t from one end, A2 represents an area ratio of the amorphousoxide matrix in a longitudinal range of ⅓×t from the other end, and trepresents the length of the plug in a direction perpendicular to thelongitudinal direction of the plug.

The ratio A1/A2 of the area ratios A1 and A2 of the amorphous oxidematrix preferably meets the relation of ⅔≦A1/A2≦1.5.

The amorphous oxide matrix is preferably silica.

The method of the present invention for producing a ceramic honeycombfilter comprising a ceramic honeycomb structure having pluralities offlow paths partitioned by porous cell walls made of a materialcomprising cordierite as a main crystal, and plugs formed inpredetermined flow paths of the ceramic honeycomb structure; comprising

charging a plugging material comprising at least 100 parts by mass ofceramic particles, 5-20 parts by mass on a solid basis of colloidaloxide and 1.5-4 parts by mass of a binder, into the predetermined flowpaths of the ceramic honeycomb structure, and then drying the pluggingmaterial by microwave heating or high-frequency dielectric heating toform the plugs; and

the ceramic particles having a particle size distribution having atleast a first peak, and a second peak lower than the first peak, thefirst peak being in a particle size range of 100-200 μm, and the secondpeak being in a particle size range of 10-30 μm.

Before the plugging material charged into the predetermined flow pathsof the ceramic honeycomb structure is subjected to microwave heating orhigh-frequency dielectric heating, an end surface of the ceramichoneycomb structure on the plugging-material-charged side is preferablybrought into contact with a heat-conducting means for preheating at30-80° C. for 1-10 minutes.

The ceramic particles are preferably obtained by mixing 20-50% by massof first ceramic particles having an average particle size of 90-200 μmwith 50-80% by mass of second ceramic particles having an averageparticle size of 5-30 μm.

The microwave heating is preferably conducted by irradiating theplugging material with microwave having power of 1-30 W/g per a unitmass of the plugging material for 1-20 minutes.

The high-frequency dielectric heating is preferably conducted byhigh-frequency electric power of 1-20 W/g per a unit mass of theplugging material for 1-5 minutes, with a distance of 1-15 mm betweenthe end surface of the ceramic honeycomb structure and a high-frequencypower electrode.

The colloidal oxide is preferably colloidal silica.

The ceramic material powder is preferably cordierite powder.

Effects of the Invention

The ceramic honeycomb filter of the present invention has excellentbonding strength between plugs and cell walls, because there is no largedifference in the concentration of colloidal oxide between the flow pathend surface side and opposite side of the plugs. The method of thepresent invention can produce a ceramic honeycomb filter having highbonding strength between plugs and cell walls, even when the plugs arebonded at a low temperature. The lower bonding temperature of plugsprovides drastic production cost reduction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a perspective view schematically showing an example of theceramic honeycomb filters of the present invention.

FIG. 1(b) is a view showing a longitudinal cross section of the ceramichoneycomb filter of the present invention shown in FIG. 1(a)

FIG. 2(a) is a schematic view showing a plugging step in the productionmethod of a ceramic honeycomb filter.

FIG. 2(b) is a schematic view showing another plugging step in theproduction method of a ceramic honeycomb filter.

FIG. 2(c) is a schematic view showing a further plugging step in theproduction method of a ceramic honeycomb filter.

FIG. 3 is a schematic view showing the positions of measuring the arearatio of an amorphous oxide matrix in a plug in the ceramic honeycombfilter.

FIG. 4 is a graph showing the particle size distribution of ceramicmaterial powder used in Example 3 of the present invention.

FIG. 5 is an electron photomicrograph showing a cross section of a plugin the ceramic honeycomb filter produced in Example 1 of the presentinvention.

FIG. 6 is an electron photomicrograph showing a cross section of a plugin the ceramic honeycomb filter produced in Example 1 of the presentinvention.

FIG. 7 is an electron photomicrograph showing a cross section of a plugin the ceramic honeycomb filter produced in Example 1 of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be specifically explainedbelow without intention of restricting the present invention thereto. Itshould be noted that proper modifications and improvements can be madebased on the usual knowledge of those skilled in the art within thescope of the present invention.

[1] Ceramic Honeycomb Filter

The ceramic honeycomb filter of the present invention comprises aceramic honeycomb structure having pluralities of flow paths partitionedby porous cell walls made of a material comprising cordierite as a maincrystal, and plugs formed in predetermined flow paths of the ceramichoneycomb structure;

the plugs comprising ceramic particles and an amorphous oxide matrixexisting between the ceramic particles;

the amorphous oxide matrix being 5-20 parts by mass per 100 parts bymass of the ceramic particles; and

in a cross section of the plug including the center axis of the flowpath, the plugs having a ratio of A1/A2 meeting the relation of½≦A1/A2≦2, wherein A1 represents an area ratio of an amorphous oxidematrix in a longitudinal range of ⅓×t from one end, A2 represents anarea ratio of the amorphous oxide matrix in a longitudinal range of ⅓×tfrom the other end, and t represents the length of the plug in adirection perpendicular to the longitudinal direction of the plug. Inthe case of a honeycomb having a square or hexagonal lattice shape, “t”corresponds to the distance between opposing cell walls. In the case ofa honeycomb having a triangular lattice shape, “t” corresponds to theheight of a triangle. “t” may be called the width of plug below.

When the plugs meet the above requirement, namely when there is smalldifference between the concentration of an amorphous oxide matrix in arange of ⅓×t from one end of the plug (for example, plug end on the flowpath end surface side), and the concentration of an amorphous oxidematrix in a longitudinal range of ⅓×t from the other end of the plug(for example, on the flow path inner side), wherein t represents thelength of the plug in a direction perpendicular to the longitudinaldirection of the plug, the plugs have good bonding strength to cellwalls in their entire length from one end to the other end, so that theplugs are not easily detached during use, resulting in high resistanceto particulate-matter-capturing performance decrease. The ratio A1/A2 ofthe area ratios A1 and A2 of an amorphous oxide matrix meets therelation of preferably ⅔≦A1/A2≦1.5, more preferably 0.8≦A1/A2≦1.3.

The area ratios A1 and A2 of an amorphous oxide matrix in aflow-path-direction, center-axis-including cross section of a plug canbe determined, for example, as follows. Namely, electronphotomicrographs (FIGS. 6 and 7) taken on a flow-path-direction,center-axis-including cross section of a plug in the ceramic honeycombfilter are analyzed by an image analyzer (for example, Image-Pro Plusver. 7.0 available from Media Cybernetics). In the electronphotomicrographs of FIGS. 6 and 7 having black portions,high-concentration gray portions, and low-concentration gray portions,it is confirmed by EDX composition analysis that the high-concentrationgray portions (shown by the arrow a) are an amorphous oxide matrix(SiO₂), the low-concentration gray portions (shown by the arrow b) areaggregate (cordierite 5SiO₂.2Al₂O₃.2MgO), and the black portions (shownby the arrow c) are voids. The area of the amorphous oxide matrix (forexample, portion shown by the arrow) is determined from the photograph,and divided by a field area to obtain the area ratio. As shown in FIG.3, the area ratio A1 of an amorphous oxide matrix in a range a1corresponding to ⅓ of the width t of the plug 13 a from one end 131 a(an end of a plug on the flow path end surface side in the figure), andthe area ratio A2 of an amorphous oxide matrix in a range a2corresponding to ⅓ of the width t of the plug 13 a from the other end132 a (an end of a plug inside a flow path in the figure) aredetermined.

In the ceramic honeycomb filter of the present invention, the amorphousoxide matrix is preferably silica. When the amorphous oxide matrix issilica, the plugs have such high bonding strength to the cell walls thatthey are less detachable during use, avoiding decrease inparticulate-matter-capturing performance. The amorphous oxide matrix ispreferably made of colloidal oxide. The colloidal oxide is preferablycolloidal silica.

[2] Production Method of Ceramic Honeycomb Filter

The production method of the ceramic honeycomb filter of the presentinvention will be explained below. The ceramic honeycomb filter forremoving particulate matter from an exhaust gas is produced by formingplugs by charging a plugging material into predetermined flow paths of aceramic honeycomb structure having pluralities of flow paths partitionedby porous cell walls made of a material comprising cordierite as a maincrystal. The plugging material comprises at least 100 parts by mass ofceramic particles, 5-20 parts by mass on a solid basis of colloidaloxide, and 1.5-4 parts by mass of a binder. The ceramic particles have aparticle size distribution having at least a first peak, and a secondpeak lower than the first peak, the first peak being in a particle sizerange of 100-200 μm, and the second peak being in a particle size rangeof 10-30 μm.

A method of charging a plugging material into predetermined flow pathsof a ceramic honeycomb structure will be explained referring to FIG. 2.Plugging films 21 a, 21 b are attached to end surfaces 12 a, 12 b of aceramic honeycomb structure 10, and provided with penetrating pores 22at positions corresponding to the flow paths 15 a or 15 b, for example,by laser irradiation [FIG. 2(a)]. The plugging films 21 a, 21 b areprovided with penetrating pores 22 in a checkerboard pattern, such thatthe flow paths 15 a are provided with plugs 13 a on the side of the endsurface 12 a while being open on the side of the end surface 12 b, andthat the flow paths 15 b are provided with plugs 13 b on the side of theend surface 12 b while being open on the side of the end surface 12 a.The penetrating pores 22 can be formed by piercing the plugging filmwith a sharp-pointed metal pin, or by pushing a heated metal pin to theplugging film, as long as openings are formed in the plugging film.

The ceramic honeycomb structure 10 is then immersed in a slurry of theplugging material 23 on the side of the end surface 12 a, so that theplugging material 23 is introduced into the flow paths 15 a through thepenetrating pores 22 formed in the plugging film 21 a [FIG. 2(b)]. Tohave fluidity for easy charging, the plugging material comprising atleast ceramic material powder and colloidal oxide contains water.

The plugging material charged into the predetermined flow paths of theceramic honeycomb structure is dried by microwave heating orhigh-frequency dielectric heating, so that the plugging material isbonded to the ceramic honeycomb structure. In the plugging materialcomprising at least ceramic particles, colloidal oxide, a binder andwater, the colloidal oxide irreversibly forms a strong sold, anamorphous oxide matrix, by dehydration, thereby bonding ceramicparticles. Microwave heating or high-frequency dielectric heating may beconducted, after the plugging material is charged into one or both endsof the predetermined flow paths of the ceramic honeycomb structure.

By heating the plugging material by microwave heating or high-frequencydielectric heating, the entire plugs are uniformly heated withouttemperature gradient. Because a liquid component is evaporated from theplugging material by such uniform heating, not only from one end ofplugs (for example, ends of plugs on the flow path end surface side),but also from the other end (for example, ends of plugs inside flowpaths), and partially through cell walls, the phenomenon that colloidaloxide is segregated in one-side portions as in the drying of plugs in ahot-air furnace does not occur, resulting in a smaller concentrationdifference of colloidal oxide longitudinally in plugs. Accordingly,plugs are well bonded to cell walls at any longitudinal position. Thus,drying by microwave heating or high-frequency dielectric heatingprovides a ceramic honeycomb filter having a ratio A1/A2 meeting therelation of ½≦A1/A2≦2, wherein A1 represents an area ratio of anamorphous oxide matrix in a longitudinal range of ⅓×t from one end, andA2 represents an area ratio of an amorphous oxide matrix in alongitudinal range of ⅓×t from the other end, in a cross section of theplug including the center axis of the flow path; and t represents thelength of plug in a direction perpendicular to the longitudinaldirection. An area ratio A3 of an amorphous oxide matrix in anintermediate portion between one end and the other end meets ⅓×A1<A3≦A1,and ⅓×A2<A3≦A2.

Before microwave heating or high-frequency dielectric heating, thecharged plugging material is preferably preheated. The preheating ispreferably started within 5 minutes after charging, and conducted at30-80° C. for 1-10 minutes with the charged plugging material in contactwith a heat-conducting means. The preheating is conducted, for example,by bringing an end surface of the ceramic honeycomb structure on theplugging-material-charged side into contact with an electric heatingplate, etc. heated to a predetermined temperature. The preheatingtemperature is preferably 35-70° C., more preferably 40-60° C. With theplugging material preheated, the binder in the plugging material isgelled (hardened), lowering the fluidity of a plugging material slurry,thereby preventing the end surface of the plugging material from beingdented on the charged side. Thus, the resultant plugs have enough lengthand high strength. The binder is preferably thermally hardenable bygelation, particularly methylcellulose, etc. Though the preheating of anend surface of the ceramic honeycomb structure may be conducted bydirect contact of the end surface with an electric heating plate, etc.,it is preferably conducted, for example, with a paper or a clotharranged between them, because part of the plugging material remainsattached to the electric heating plate, failing to have enough pluglength.

The microwave irradiation is preferably conducted at 1-30 W/g per a unitmass of the plugging material for 1-20 minutes. The high-frequencydielectric heating is preferably conducted by high-frequency power of1-20 W/g per a unit mass of the plugging material for 1-5 minutes, withhigh-frequency voltage-applying electrodes and ground electrodesalternately arranged with predetermined intervals placed 1-15 mmseparate from the end surfaces of the ceramic honeycomb structure. Theplugging material is heated to about 80-200° C. by microwave heating orhigh-frequency dielectric heating under such conditions, resulting inhigher bonding strength of the plugs to the cell walls. Accordingly,even when the plugs are formed at 1000° C. or lower, the plugs are wellbonded to the cell walls, resistant to detachment during use, resultingin a ceramic honeycomb filter free from decrease inparticulate-matter-capturing performance. The gap between an end surfaceof the ceramic honeycomb structure and each high-frequency electrode maybe determined by placing a ceramic plate having a desired thickness onthe high-frequency electrode, and placing the ceramic honeycombstructure thereon.

The microwave heating or the high-frequency dielectric heating ispreferably started within 20 minutes after the plugging material ischarged into the predetermined flow paths of the ceramic honeycombstructure, when no preheating is conducted. When 20 minutes or more passwithout preheating after charging, a liquid component in the pluggingmaterial is absorbed into the cell walls by capillary phenomenon.Accordingly, colloidal oxide in the plugging material likely migrates tothe cell walls together with water, resulting in lower bonding strengthof the plugs to the cell walls on the flow path end surface side, andlower strength of the plugs per se. As a result, the plugs are likelydetached during use, resulting in lower particulate-matter-capturingperformance. The microwave heating or high-frequency dielectric heatingis more preferably conducted within 10 minutes after the pluggingmaterial is charged. While the microwave heating takes a long dryingtime because the entire honeycomb body including plugs is heated in amicrowave apparatus, the high-frequency dielectric heating canefficiently heat the plugs only. When preheating is conducted after theplugging material is charged into the predetermined flow paths of theceramic honeycomb structure, the binder in the plugging material isgelled. Because water is still contained in the plugging material, highbonding strength of the plugs can be obtained when the microwave heatingor high-frequency dielectric heating is started within 60 minutes afterpreheating.

The ceramic particles have a particle size distribution having at leasta first peak, and a second peak lower than the first peak, the firstpeak being in a particle size range of 100-200 μm, and the second peakbeing in a particle size range of 10-30 μm. Namely, the particle sizedistribution of the ceramic particles has at least two peaks, a higherpeak being called “first peak,” and a peak lower than the first peakbeing called “second peak.” Such particle size distribution means thatceramic particles in the plugging material comprise at least two typesof powder, powder having larger particle sizes and powder having smallerparticle sizes. Using ceramic particles having such a particle sizedistribution, ceramic particles having smaller particle sizes intrudespace between ceramic particles having larger particle sizes, resultingin plugs having a high filling density of ceramic particles.Accordingly, the plugs are then strongly bonded to the cell walls at alow temperature, resulting in a ceramic honeycomb filter suffering nodecrease in particulate-matter-capturing performance, with plugs lessdetachable during use.

The ceramic particles having such a particle size distribution ispreferably obtained by mixing 20-50% by mass of first ceramic particleshaving an average particle size of 90-200 μm and 50-80% by mass ofsecond ceramic particles having an average particle size of 5-30 μm. Themixing of two types of such ceramic particles can provide ceramicparticles having a particle size distribution having at least a firstpeak, and a second peak lower than the first peak, the first peak beingin a particle size range of 100-200 μm, and the second peak being in aparticle size range of 10-30 μm.

When the first ceramic particles have an average particle size of lessthan 90 μm, there are likely gaps between heat-dried plugs and cellwalls, so that the plugs are easily detachable, resulting in lowparticulate-matter-capturing performance. When the first ceramicparticles have an average particle size of more than 200 μm, there islikely a higher percentage of powder having larger particle sizes,resulting in low heat shock resistance. The first ceramic particlespreferably have an average particle size of 100-180 μm.

When the second ceramic particles have an average particle size of lessthan 5 μm, there is likely a higher percentage of powder having largerparticle sizes, resulting in low heat shock resistance. When the secondceramic material powder has an average particle size of more than 30 μm,the heat-dried plugs likely have voids, so that the plugs are easilydetachable, having low particulate-matter-capturing performance. Thesecond ceramic particles preferably have an average particle size of10-25 μm.

When the amount of the first ceramic particles mixed is less than 20%(when the amount of the second ceramic particles mixed is more than80%), the heat-dried plugs likely have voids, so that the plugs areeasily detachable, having low particulate-matter-capturing performance.On the other hand, when the amount of the first ceramic particles mixedis more than 50% (when the amount of the second ceramic particles mixedis less than 50%), there is likely a higher percentage of powder havinglarger particle sizes, resulting in low heat shock resistance. Theamounts of the first and second ceramic particles are more preferably25-45% of the first ceramic particles and 55-75% of the second ceramicparticles.

The particle size distribution of ceramic particles can be measured by aparticle size distribution meter (Microtrack MT3000 available fromNikkiso Co., Ltd.). In FIG. 4, the axis of abscissa represents aparticle size, and the axis of ordinates represents the frequency (%) ofeach particle size. The second peak lower than the first peak means thatthe frequency P2 (height) of the second peak is smaller than thefrequency P1 (height) of the first peak. When the plugs are formed,ceramic particles having smaller particle sizes intrude gaps betweenthose having larger particle sizes, resulting in a higher filling ratio.To have high bonding strength between the low-temperature-bonded plugsto the cell walls, the height P1 of the first peak is preferably 3 timesor less, more preferably 2 times or less, of the height P2 of the secondpeak.

The first ceramic particles preferably has sphericity of 0.6 or more.The first ceramic particles having sphericity of 0.6 or more have smallsurface areas, so that they are easily bonded to the second ceramicparticles, preferably resulting in high bonding strength of the pluggingmaterial and between the plugs and the cell walls. The sphericity of thefirst ceramic particles is preferably 0.7 or more, more preferably 0.8or more. The sphericity is determined by dividing the area of eachprojected image of 10 particles measured by image analysis on anelectron photomicrograph, by the area of a circle having a diametercorresponding to the maximum length between two points at which astraight line passing a center of gravity of each particle crosses acircumference of the particle, and averaging the calculated ratios for10 particles.

With the plugs fixated by cordierite-based ceramic particles at 1000° C.or lower in the production of the ceramic honeycomb filter of thepresent invention, the difference in a thermal expansion coefficientbetween the ceramic honeycomb structure and the plugs can be made small,resulting in a ceramic honeycomb filter having good heat shockresistance. The first ceramic particles are preferably sintered porouscordierite powder. The porous cordierite powder preferably has porosityof 40-60%.

The present invention will be explained in more detail by Examples belowwithout intention of restriction.

Example 1

Kaolin powder, talc powder, silica powder and alumina powder were mixedto prepare cordierite-forming material powder comprising 50% by mass ofSiO₂, 35% by mass of Al₂O₃, and 13% by mass of MgO, which was then fullymixed with a binder such as methylcellulose, hydroxypropylmethylcellulose, etc., a lubricant, and hollow resin balloons as apore-forming material in a dry state. With a predetermined amount ofwater added, they were sufficient blended to prepare a plasticizedmoldable ceramic material. The moldable material was extruded, and cutto a honeycomb-structured green body of 270 mm in diameter and 300 mm inlength. The green body was dried and sintered to obtain acordierite-type ceramic honeycomb structure 10 having a cell wallthickness of 0.3 mm, a cell wall pitch of 1.5 mm, porosity of 63%, andan average pore size of 21 μm.

As shown in FIG. 2, a plugging resin film of 0.09 mm in thickness wasattached to each of both ground end surfaces 12 a, 12 b of the ceramichoneycomb structure 10, and each plugging resin film was provided withpenetrating pores at positions corresponding to flow paths to be pluggedin a checkerboard pattern by laser beams [FIG. 2(a)]. The penetratingpores 22 of the plugging films 21 a, 21 b were formed in a checkerboardpattern, such that flow paths 15 b were open at the end surface 12 a,and flow paths 15 a were open at the end surface 12 b.

As shown in Table 1, 100 parts by mass of ceramic material powderobtained by mixing the first ceramic particles and the second ceramicparticles (both made of cordierite) was mixed and blended with colloidaloxide (colloidal silica having a solid concentration of 40% by mass) inan amount shown in Table 2, 50 parts by mass of ion-exchanged water, 2.5parts by mass of methylcellulose as a binder, to prepare a pluggingmaterial slurry. The particle size distribution of the ceramic materialpowder used was measured by a particle size distribution meter(Microtrack MT3000 available from Nikkiso Co., Ltd.), to determine thefrequency P1 (height) of the first peak and the frequency P2 (height) ofthe second peak.

TABLE 1 First Ceramic Particles ⁽¹⁾ Second Ceramic Particles ⁽²⁾ AverageAmount Average Amount Particle (parts Particle (parts Size by Size byNo. (μm) Sphericity mass) (μm) Sphericity mass) Example 1 125 0.8 3213.5 0.6 68 Example 2 125 0.8 40 13.5 0.6 60 Example 3 125 0.8 23 13.50.6 77 Example 4 190 0.7 31 27 0.5 69 Example 5 125 0.8 32 13.5 0.6 68Example 6 125 0.8 23 13.5 0.6 77 Com. Ex. 1 25 0.8 100 — — — Com. Ex. 2167 0.6 5 13 0.6 95 Com. Ex. 3 250 0.4 33 36 0.4 67 Com. Ex. 4 25 0.8100 — — — Mixed Ceramic Material Powder Particle Size at Particle Sizeat First Peak (P1) Second Peak (P2) Peak Height Ratio No. (μm) (μm)(P1/P2) Example 1 141 19.5 1.3 Example 2 141 19.5 1.6 Example 3 141 19.51.2 Example 4 183 27 1.4 Example 5 141 19.5 1.3 Example 6 141 19.5 1.2Com. Ex. 1 25 — — Com. Ex. 2 15 160 1.2 Com. Ex. 3 235 33 1.3 Com. Ex. 425 — — Note: ⁽¹⁾ Cordierite particles. ⁽²⁾ Cordierite particles.

TABLE 2 Formulation (parts by mass) Ceramic Material No. Powder ⁽¹⁾Colloidal Oxide ⁽²⁾ Example 1 100 40 [16] Example 2 100 40 [16] Example3 100 40 [16] Example 4 100 37.5 [15]  Example 5 100 40 [16] Example 6100 40 [16] Com. Ex. 1 100 40 [16] Com. Ex. 2 100 40 [16] Com. Ex. 3 10037.5 [15]  Com. Ex. 4 100 40 [16] Note: ⁽¹⁾ A mixture of the firstceramic particles and the second ceramic particles. ⁽²⁾ Colloidal silicahaving a solid concentration of 40% by mass.

The end surface 12 a of the ceramic honeycomb structure 10 was immersedin a bath of a plugging material 23, which was introduced into the flowpaths 15 a to the depth of 10 mm through penetrating pores 22 formed inthe plugging film 21 a [FIG. 2(b)]. Immediately after introducing theplugging material 23, the end surface 12 a on the side of which theplugging material 23 was introduced was preheated via four papers on anelectric heating plate at 50° C. for 5 minutes. Another end surface 12 bof the ceramic honeycomb structure 10 was then immersed in a bath of theplugging material 23, which was similarly introduced into the flow paths15 b to the depth of 10 mm through penetrating pores 22 formed in theplugging film 21 b. The end surface 12 b was also preheated by a hotplate like the end surface 12 a. With the plugging films 21 a, 21 bpeeled, the plugs were heated by microwave of 2450 MHz having power of12 W/g per a unit mass of the plugging material in a microwave heatingapparatus for 4 minutes (see Table 3), to dry the plugging material 23,thereby producing the of ceramic honeycomb filter of Example 1.

TABLE 3 Heating Conditions Maximum Time Until Heating Was Started AfterHeating No. Preheating Charged Heating Method Time Example 1 50° C., 13minutes Microwave 4 minutes 5 minutes (12 W/g) Example 2 50° C., 13minutes microwave 4 minutes 5 minutes (12 W/g) Example 3 50° C., 13minutes Microwave 4 minutes 5 minutes (12 W/g) Example 4 50° C., 15minutes Microwave 4 minutes 5 minutes (12 W/g) Example 5 No 8 minutesMicrowave 4 minutes (12 W/g) Example 6 50° C., 13 minutes High-Frequency1 minute 5 minutes (6.5 W/g)  Com. Ex. 1 50° C., 35 minutes Hot AirFurnace 3 hours 5 minutes (500° C.) Com. Ex. 2 50° C., 35 minutes HotAir Furnace 3 hours 5 minutes (500° C.) Com. Ex. 3 50° C., 13 minutesMicrowave 4 minutes 5 minutes (12 W/g) Com. Ex. 4 50° C., 13 minutesMicrowave 4 minutes 5 minutes (12 W/g)

Examples 2-4

The ceramic honeycomb filters of Examples 2-4 were produced in the samemanner as in Example 1, except that the types and amounts of the ceramicmaterial powder and colloidal oxide (colloidal silica having a solidconcentration of 40% by mass) were changed as shown in Tables 1 and 2,and that the heating conditions were changed as shown in Table 3.

Example 5

A ceramic honeycomb structure 10 was produced in the same manner as inExample 1, and the plugging material 23 was introduced into the ceramichoneycomb structure 10 on the side of the end surface 12 a. The endsurface 12 a on the side of which the plugging material 23 wasintroduced was not preheated. The plugging material 23 was thenintroduced into another end surface 12 b of the ceramic honeycombstructure 10 as in Example 1, with the end surface 12 b similarly notpreheated. With the plugging films 21 a, 21 b peeled, microwave heatingwas conducted in the same manner as in Example 1 to produce the ceramichoneycomb filter of Example 5.

Example 6

The ceramic honeycomb filter of Example 6 was produced in the samemanner as in Example 3, except that the plugs were heated byhigh-frequency (40 MHz) power of 6.5 W/g per a unit mass of the pluggingmaterial at the distance of 3 mm from an end surface of the ceramichoneycomb structure for 1 minute by a high-frequency heating apparatus,in place of the microwave heating (see Table 3).

Comparative Examples 1-4

The ceramic honeycomb filters of Comparative Examples 1-3 were producedin the same manner as in Example 1, except that the types and amounts ofceramic material powders and the amount of colloidal oxide (colloidalsilica having a solid concentration of 40% by mass) were changed asshown in Tables 1 and 2, and that the heating conditions were changed asshown in Table 3. The ceramic honeycomb filter of Comparative Example 1produced by the method described in JP 2005-125318 A had a particle sizedistribution having one peak, because ceramic material powder comprisingonly one type of ceramic particles was used as aggregate (see Table 1).

With respect to the plugs in the ceramic honeycomb filters of Examplesand Comparative Examples, the area ratios of amorphous oxide matrices,the bonding strength of plugs, soot-capturing performance and heat shockresistance were elevated as follows. The results are shown in Table 4.

(1) Area Ratio of Amorphous Oxide Matrix in Plugs

An electron photomicrograph of a cross section of a plug including thecenter axis of a flow path was analyzed by an image analyzer (Image-ProPlus ver. 6.3 available from Media Cybernetics) to measure the areas ofaggregate and an amorphous oxide matrix, from which an area ratio A1 ofan amorphous oxide matrix in a range a1 corresponding to ⅓ of the widtht of the plug from one end 131 a (ends of the plug on the flow path endsurface side in the figure), and an area ratio A2 of an amorphous oxidematrix in a range a2 corresponding to ⅓ of the width t of the plug fromthe other end 132 a (ends of the plug inside the flow path in thefigure), to calculate a ratio A1/A2, as shown in FIG. 3. Further, anarea ratio A3 of an amorphous oxide matrix in an intermediate portionbetween one end 131 a and the other end 132 a of plugs 13 a (at centerof a range a3 between the range a1 and the range a2) was determined.

(2) Strength of Plugs

The bonding strength of plugs to cell walls was determined by pushing aflat-tipped push rod having a diameter of 0.8 mm to a plug, measuring aload when the push rod crashed the plug, or when the plug was detached,dividing the load by a cross section area (2.01 mm²) of the push rod tocalculate the strength (MPa) of each plug, and averaging the strengthvalues measured on 10 plugs. The results are shown in Table.

(3) Soot-Capturing Performance

With carbon powder having a particle size of 0.042 μm introduced into aceramic honeycomb filter at a speed of 3 g/h together with air flow of10 Nm³/min in a pressure loss test stand, the number Nin of carbonpowder particles flowing into the honeycomb filter and the number Noutof carbon powder particles flowing out of the honeycomb filter werecounted by Model 3936 of a scanning mobility particle sizer (SMPS)available from TIS, for 1 minute between 3 minutes and 4 minutes afterstart, to calculate the capturing ratio of soot by the formula of (NinNout)/Nin. Soot-capturing performance was evaluated by the followingstandard:

-   -   Excellent: The capturing ratio was 98% or more,    -   Good: The capturing ratio was 95% or more and less than 98%,    -   Fair: The capturing ratio was 90% or more and less than 95%, and    -   Poor: The capturing ratio was less than 90%.

(4) Heat Shock Resistance

The evaluation test of heat shock resistance was conducted by heatingthe ceramic honeycomb filter at 400° C. for 30 minutes in an electricfurnace, rapidly cooling it to room temperature, and observing cracks incell walls near the plugs by the naked eye. When no cracks wereobserved, the same test was conducted with the temperature of theelectric furnace elevated by 25° C., and this operation was repeateduntil cracking occurred. The test was conducted three times for eachsample. The difference between a temperature at which cracking occurredin at least one honeycomb structure and room temperature (heatingtemperature−room temperature) was regarded as a heat shock resistancetemperature, which was evaluated by the following standard:

-   -   Excellent: The heat shock resistance temperature was 550° C. or        higher,    -   Good: The heat shock resistance temperature was 500° C. or        higher and lower than 550° C.,    -   Fair: The heat shock resistance temperature was 450° C. or        higher and lower than 500° C., and    -   Poor: The heat shock resistance temperature was lower than 450°        C.

(5) Porosity of Plugs

The porosity of a peripheral wall was determined by analyzing anelectron photomicrograph of a cross section of a plug cut out of theceramic honeycomb filter by an image analyzer (Image-Pro Plus ver. 7.0of Media Cybernetics).

TABLE 4 Amorphous Oxide Matrix Evaluation Results in Plug PorosityBonding Heat Area Ratio A1/ of Plug Strength Capturing Shock No. A1 A2A3 A2 (%) (MPa) Ratio Resistance Example 1 42 40 17 1.1 28 51 ExcellentExcellent Example 2 40 34 15 1.2 31 45 Excellent Good Example 3 41 35 181.2 27 34 Good Excellent Example 4 40 32 19 1.3 30 34 Good ExcellentExample 5 42 40 13 1.1 29 29 Excellent Excellent Example 6 41 35 15 1.228 34 Good Excellent Com. Ex. 1 45 8 9 5.6 38 22 Poor Good Com. Ex. 2 4315 17 2.9 24 24 Poor Good Com. Ex. 3 42 19 21 2.2 43 23 Fair Fair Com.Ex. 4 43 18 20 2.4 32 24 Poor Good

FIG. 5 is an electron photomicrograph of a cross section of a plug inthe ceramic honeycomb filter of Example 1 of the present invention,indicating that the plugs produced by the method of the presentinvention were uniform with little segregation of colloidal oxide. It isclear from Table 4 that the ceramic honeycomb filters of Examples 1-6 ofthe present invention had excellent bonding strength of plugs,soot-capturing performance and heat shock resistance, though theplug-bonding strength was slightly poor in Example 5, in whichpreheating was not conducted.

On the other hand, in any of Comparative Example 1 in which ceramicparticles having a particle size distribution having only one peak at 25μm was used for a plugging material which was dried in a hot-airfurnace, Comparative Example 4 in which a plugging material comprisingthe same ceramic particles as in Comparative Example 1 was dried bymicrowave, and Comparative

Example 2 in which ceramic particles having two peaks at 15 μm (firstpeak) and at 160 μm (second peak), the first peak being outside theparticle size range of 100-200 μm, and the second peak being outside theparticle size range of 10-30 μm was used for a plugging material whichwas dried in a hot-air furnace, the plug-bonding strength and thesoot-capturing performance were extremely poor. In Comparative Example 3in which ceramic particles having a first peak outside the particle sizerange of 100-200 μm and a second peak outside the particle size range of10-30 μm, the plug-bonding strength was extremely poor, and thesoot-capturing performance and the heat shock resistance were slightlypoor.

1. A ceramic honeycomb filter comprising a ceramic honeycomb structure having pluralities of flow paths partitioned by porous cell walls made of a material comprising cordierite as a main crystal, and plugs formed in predetermined flow paths of said ceramic honeycomb structure; said plugs comprising ceramic particles and an amorphous oxide matrix existing between said ceramic particles; said amorphous oxide matrix being 5-20 parts by mass per 100 parts by mass of said ceramic particles; and in a cross section of said plug including the center axis of said flow path, a ratio A1/A2 meeting the relation of ½≦A1/A2≦2, wherein A1 represents an area ratio of said amorphous oxide matrix in a longitudinal range of ⅓×t from one end, A2 represents an area ratio of said amorphous oxide matrix in a longitudinal range of ⅓×t from the other end, and t represents the length of said plug in a direction perpendicular to the longitudinal direction of said plug.
 2. The ceramic honeycomb filter according to claim 1, wherein the ratio A1/A2 of area ratios A1 and A2 of said amorphous oxide matrix meets the relation of ⅔≦A1/A2≦1.5.
 3. The ceramic honeycomb filter according to claim 1, wherein said amorphous oxide matrix is silica.
 4. A method for producing a ceramic honeycomb filter comprising a ceramic honeycomb structure having pluralities of flow paths partitioned by porous cell walls made of a material comprising cordierite as a main crystal, and plugs formed in predetermined flow paths of said ceramic honeycomb structure; comprising charging a plugging material comprising at least 100 parts by mass of ceramic particles, 5-20 parts by mass on a solid basis of colloidal oxide and 1.5-4 parts by mass of a binder into the predetermined flow paths of said ceramic honeycomb structure, and drying said plugging material by microwave heating or high-frequency dielectric heating to form said plugs; said ceramic particles having a particle size distribution at least a first peak and a second peak lower than said first peak, said first peak being in a particle size range of 100-200 μm, and said second peak being in a particle size range of 10-30 μm.
 5. The method for producing a ceramic honeycomb filter according to claim 4, wherein after said plugging material is charged into the predetermined flow paths of said ceramic honeycomb structure, and before said microwave heating or high-frequency dielectric heating is conducted, an end surface of said ceramic honeycomb structure on the side that said plugging material is charged is preheated at 30-80° C. for 1-10 minutes with a heat-conducting means contacted.
 6. The method for producing a ceramic honeycomb filter according to claim 4, wherein said ceramic particles comprise 20-50% by mass of first ceramic particles having an average particle size of 90-200 μm, and 50-80% by mass of second ceramic particles having an average particle size of 5-30 μm.
 7. The method for producing a ceramic honeycomb filter according to claim 4, wherein said microwave heating is conducted by irradiating microwave with power of 1-30 W/g per a unit mass of said plugging material for 1-20 minutes.
 8. The method for producing a ceramic honeycomb filter according to claim 4, wherein said high-frequency dielectric heating is conducted by applying a high-frequency electric field with power of 1-20 W/g per a unit mass of said plugging material at a distance of 1-15 mm between an end surface of said ceramic honeycomb structure and a high-frequency electrode, for 1-5 minutes.
 9. The method for producing a ceramic honeycomb filter according to claim 4, wherein said colloidal oxide is colloidal silica.
 10. The method for producing a ceramic honeycomb filter according to claim 4, wherein said ceramic particles are based on cordierite.
 11. The method for producing a ceramic honeycomb filter according to claim 4, wherein said plugs are formed without sintering. 